The present invention relates to a short-time arc welding system for welding elements, such as, for example metal studs, onto components, such as, for example metal sheets, with:                a robot, having at least one arm movable in at least two coordinate axes,        a welding head base fixed to the robot arm,        a welding head, mounted movably on the welding head base, on which a holding device for holding an element and a lifting device for advancing and returning the holding device relative to the welding head are provided, and        a measuring system for determining the relative position between a component and an element to be welded onto the component.        
The invention further relates to a method for short-time arc welding, in particular for stud welding, of elements, such as, for example metal studs, onto components, such as, for example metal sheets. A short-time arc welding system of this kind and an associated method are generally known.
With short-time arc welding an element is welded onto a component. An arc is therein formed between element and component, which melts the end faces. Then the element and the component are moved towards one another, so the molten masses are mixed. The arc is short-circuited and the combined molten mass solidified.
It is usual to draw the arc. The element is therein first placed onto the component. Then a pilot current is switched on and the element is lifted with respect to the component up to a desired height, wherein an arc is drawn. Only after this is the welding current connected.
In order to achieve consistently good welding results, it is important to know, inter alia, the relative position between element and component, in particular to lift the element to the right height, before the welding current is connected. For this purpose the relative position is usually measured before each welding process, in particular in the form of determining a zero position.
This is of particular significance with robot-based systems. Though modern robots are generally capable of positioning comparatively accurately, it is not possible to achieve great precision with very high dynamics, in particular owing to the large masses moved.
Stud welding systems are used in particular in the automotive industry. There they serve, above all, to weld elements such as studs with and without thread, eyes, nuts, etc., onto the vehicle body panel. These elements then serve as holding anchors to fix, for example interior fittings.
In the automotive industry manufacturing speed is what really matters. Within a few minutes hundreds of elements have to be welded on at different positions automatically by means of robots. The robots therefore have to be moved with high dynamics.
It is therefore known to attach to the arm of a robot a welding head base, which bears a cradle. The cradle can be moved with high dynamics with great precision, usually by means of a pneumatic or hydraulic system. On the cradle is mounted the actual welding head, which again has a lifting device for moving the element.
From DE 41 208 11 A1 a stud welding device is known, which uses a screw thread reluctance motor (SGRM) as linear motor for axially adjusting a stud holder. For this purpose the moved member of the SGRM forms an adjusting member, the movement of which is transmitted to the stud holder. The non-moved member forms the holder of the stud welding device. The SGRM allows the axial movement forwards and backwards of the stud holder to be controlled in a defined manner with respect to the setting position and speed.
In the position of rest the moved member rests against a stop connected to the holder under the tension of a spring. Only by triggering the SGRM is the moved member displaced out of the position of rest towards the workpiece which is to be provided with a welding stud. The stud holder therein takes up an intermediate position according to a predetermined path before the welding stud impacts on the workpiece, whereby by making contact the welding arc is struck in the known manner.
U.S. Pat. No. 5,252,802 discloses a further stud welding device with a housing, constructed as a hand pistol. A positioning motor first brings the housing into a position in which a stud is arranged near a component. In the housing a linear motor is provided, to move axially a lifting shank, which carries the stud. A path-measuring system is provided for controlling the linear motor. To determine the relative position between stud and workpiece the linear motor is triggered to move the stud at a certain speed towards the workpiece. As soon as the stud touches the workpiece an electrical contact closes.
It is further known from WO 96/11767 to bias the stud holder elastically towards the workpiece and by means of a linear motor to move it axially against the bias.
For detecting the relative position between stud and workpiece it is also known (for example from “Neue TUCKER Technologie. Bolzenschweiβen mit System”, Emhart Tucker, September 1999) to determine a zero position by means of a support foot.
Finally WO96/05015 discloses a stud welding device without support foot in which an entire welding head can be adjusted by means of an adjustment drive. A holding device holding a stud is provided on the welding head. An adjustment device serves to displace the holding device axially with respect to the welding head. The adjustment device can be a servo-pneumatic or a servo-hydraulic operating cylinder. The relative position between holding device and welding head is detected by means of a path measuring system.
For determining a zero position between stud and workpiece the welding head is moved towards the workpiece until it reaches an end position. In the course of this movement the stud impacts on the workpiece. As the stud from this time can no longer follow the movement of the welding head, from then on the holding device is displaced with respect to the welding head against the pressing movement. This displacement is measured by the path-measuring system and the end position of the welding head thus exactly detected.
Against the above background an object of the present invention is to cite an improved short-time arc welding system and method for short-time arc welding with which high positioning speeds are possible with great precision.
This object is achieved in a first aspect of the invention by a short-time arc welding system, as mentioned initially, wherein the measuring system has a control device which triggers the lifting device in such a way that the element is moved towards the component until it contacts the component in order thus to determine the relative position.
The object is further achieved in the first aspect of the invention by a method for short-time arc welding, in particular for stud welding, of elements, such as, for example metal studs, onto components, such as, for example metal sheets, with the steps:                a) triggering a robot with an arm in such a way that a welding head base with a welding head, fixed to the arm, goes into a basic welding position,        b) triggering a cradle bearing the welding head on the welding head base or moving a component in relation to the welding head base in such a way that the welding head goes into a head welding position,        c) triggering a lifting device of the welding head in such a way that a holding device with an element held thereon is moved towards the component, and        d) detecting the position of the lifting device in which the element contacts the component.        
According to a second aspect of the invention the object is achieved by a short-time arc welding system for welding elements, such as, for example metal studs, onto components, such as, for example metal sheets, with a welding head, on which a holding device for holding an element and a lifting device for advancing and returning the holding device relative to the welding head are provided, and a measuring system for determining the relative position between a component and an element to be welded onto the component, wherein the measuring system has a control device which triggers the lifting device in such a way that the element is moved towards the component until it contacts the component, in order thus to determine the relative position, and wherein the welding head has elastic means to bias the holding device elastically into a positioning direction.
The above object is fully achieved by the first aspect of the invention. By means of the combination of a robot and a welding head, fixed movably on the robot arm via a welding head base, high positioning speeds can be achieved with great precision. By the measure of designing the measuring system for determining the relative position between component and element in such a way that in particular no support foot is necessary, the speed can be further increased. This further has the advantage that the component is not contacted by any auxiliary means. In this respect damage of the component in the surrounding region of the welding is ruled out.
In a particularly preferred embodiment the robot arm can be moved in three coordinate axes. Robots of this kind are of particular advantage in the use of welding metal elements on to body panels of motor vehicles. For, in this field of use there is a requirement for welding the studs onto the body panel at any number of places and consequently in the most varied positions.
It is further of advantage if the welding head base has a cradle on which the welding head is mounted. A cradle, or generally speaking a device for carrying out a movement along an axis, is of advantage in so far as highly dynamic movements can be achieved therewith with comparatively great precision. It is therein particularly preferred if the cradle is pneumatically driven. It is herein of advantage that pneumatic power is usually available in the generic short-time arc welding systems as a matter of course.
In a further preferred embodiment the welding head has elastic means to bias the holding device elastically into a positioning direction. By means of this measure the lifting device for advancing and moving back the holding device can in most operating states be held without power. Consequently there is low power consumption.
This feature at the same time forms one of the core features according to the second aspect of the invention. The second aspect of the invention is directed at the design of a welding head, irrespective of whether the welding head is arranged on a robot and/or a welding head base. Decisive in the second aspect of the invention is the combination of a measuring system without a support foot with elastic means for elastic biasing of the holding device into a positioning direction.
With the second aspect of the invention it is consequently advantageous that the relative position between component and element can be measured without the component being touched by auxiliary means. The lifting device used for determining the relative position can further be held in most operating states without power, as the holding device is elastically biased in a positioning direction into a suitable position of rest.
According to a particularly preferred embodiment the elastic means bias the holding device into a return direction. As the lifting device for determining the relative position between component and element is triggered in the advancing direction, the holding device is always located in its elastically biased position of rest in the right starting position, so altogether particularly low energy consumption is achieved. Further, under certain preconditions higher dynamics can be achieved in comparison with other configurations.
In an alternative embodiment the elastic means bias the holding device in the advancing direction. In this embodiment higher dynamics can be achieved in the actual welding process in the advancing direction.
In this embodiment, when using the method according to the invention, it is so that drawing in the holding device during the approach of the welding head into the head welding position takes place against the bias force of the elastic means.
In contrast to this, in the embodiment in which the elastic means bias the holding device in the return direction it is so that the movement of the holding device in method step (c) takes place against the bias force of the elastic means.
Altogether it is preferably so in the method according to the invention that the holding device releases the element after the welding process and the lifting device is switched off, so the holding device is biased by the elastic means into a position of rest.
In the short-time arc welding system according to the invention the measuring system preferably has a path sensor which detects the path of the holding device relative to the welding head.
It goes without saying that the above mentioned features and those still to be described below can be used not only in the combination cited in each case, but also in other combinations or standing alone, without leaving the scope of the present invention.